Monthly Archives: July 2013

Really Cool Parts

Part of the fun for me when building an amp is hunting down cool parts. I try to find as many vintage parts as I can, but then I combine these with some modern items for dependability and functionality. I have scored some holy grail parts already but I still need to fill in some gaps. There is a certain mystique associated with vintage parts. Amp builders call it “mojo.” While the older parts often pale in comparison spec-wise and noise-wise to modern parts, builders use them to come as close as they can to the original tone and dare I say soul of the original Plexi and JTM45 amplifiers. The parts are expensive but my idea is to build this amp once and it will be the last amp I ever own. Here are some of the parts I have acquired:

1970s Mullard “Mustard” Capacitors – These are a must when seeking the tone, feel and look of a vintage Marshall amp. I found these N.O.S. (New Old Stock) on eBay from a seller in Greece. I still need to find a .68uF 160 volt which is pretty tough to do. They look like mustard from a squeeze bottle, hence the name.

Radiospares 560pF Capacitors – I had one of these already that I bought a long time ago, but I need 2 for the build. One is for the bright channel mix capacitor and the other for treble tone stack. I found one from a seller in France on the Metroamp Forum. paid a bunch. Ouch.

Carling Switches with Plexi-style Chrome Toggle Rings – These were both readily available on the site, a very cool place to find Marshall parts.

Piher Resistors – These are the same brand of resistors used in Marshall JMP and JCM800 amps of the late 70’s and early 80’s. I sourced them from guys in France and Spain. The 1/2 watt ones I found are from the 60s and 70s while the 2 watters are 90s vintage. I am still missing 2K7 and 4K7 values in 1/2 watt. The hunt continues…

Lemco Dogbone 47pF – This will be the phase inverter capacitor.

BC 10uF 160 volt – I found some 400 volt versions on that I was going to use but then I stumbled on the proper 160 volt versions at Valvestorm. These will be in the bias capacitor positions.

Belling Lee Fuse Holders – Very cool N.O.S. fuse holders I found on eBay from a guy in Pulborough, England.

ARS Dual Can Capacitors – These beauties will serve as the filter capacitors. When filter caps sit for a while they need to be reformed. These are sold reformed from

1K 7watt Welwyn W22 Wirewound Resistors – These will live in the screen resistor position on each power tube socket. Very cool classic resistors from Valvestorm.

Metroamp 50 Watt Plexi chassis – This build is being done on a standard 50 watt Plexi chassis. The original JCM800 amplifiers had a much longer chassis. I want to keep the build compact so I am going with a the smaller chassis. On my previous JCM800 amps I used this same sized chassis as well. Once again –

Staking Turrets

I took my perfect 9 hole high board and sliced a thin strip off to modify it to an 8 hole high which is what I had originally. On the first board I attempted, the holes were a little off, so modified this one. I carefully sanded the edge and the board was ready for turrets. I dug up my turret tools that I bought a few years ago. I bought the staking tool from Watts Tube Audio and the anvil from Hoffman Amps. I stake turrets my own way. I found the drill press method to not seat the turrets firmly enough so I use drilling vise to clamp the anvil. I set the height so that when doing turrets close to each other, the adjacent turret will not get bent.

To stake, I insert a turret into the hole I drilled in the circuit board. Then I flip the board over and place the turret into the anvil. It has a hole in it for the turret to sit in. I use a cardboard box to support the circuit board. I give the turret 4 strong taps on the back and the turret is seated.

One problem that came up was that I had some double turrets left from an old build. They use a #33 drill bit which is .113.” I knew I was going to run out of these turrets so I ordered more from Watts Tube Audio. Well, it seems the size has changed! The new double turrets they sell require a #32 bit which is .116.” You wouldn’t think 3 thousands of an inch would be that much different, but sure enough the new turrets would not press into the holes. I used a #32 bit and drilled the holes out for the new turrets. The good news is that they look almost exactly the same (the hole in the top is slightly larger – won’t matter when soldered) and I don’t have to re-stake the work I have already done.

So the lesson of the day is buy all of your turrets at the same time!

Perf Board Redo Do Over

I thought about the perf board over in over during nights of insomnia which is pretty much every night for me. I have a restless brain. I decided to return to the 8 hole high board to be more consistent with existing circuit boards of this era. Looks are important! I redid my layout for the third time. This is crazy I thought. “Can’t I just build this thing?” To me a lot of the fun is in the details. I like things just so and that sometimes requires revisiting the design. Here the latest 8 hole high layout: Screen Shot 2013-07-22 at 2.41.55 PM

Faceplate Design

Since this is going to be my JCM800 of a lifetime, I decided I am going to do custom brushed gold metal front and back plates. I am just getting my feet wet using Adobe Illustrator and it is a tricky change from my normal CAD program. Here is what I have so far. Big thanks to my drummer and best friend Tylor Durand for Illustrator advice, and Art Thompson for font consultation. The faceplate is combination of a JMP50 4 input and a JMP50 Master Volume. I am still playing with the logo and may go for a more JTM black flag look. faceplate

Perf Board Do Over

With the new design change I needed to cut and drill a new circuit board. Instead of using a plain paper template which caused misaligned holes on my last attempt, I used adhesive paper and split the design of the board into two halves. I couldn’t see paying for expensive legal adhesive paper and waiting for it to arrive in the mail. I always cut the template with a ruler an an X-ACTO knife. Scissors do not cut straight enough. I strive for precision even if the gain is small.

I stuck the two halves onto the Paxolin material. I added little pointers to the drawing to help line it up. I drilled the 288 holes very slowly this time. I also increased the speed of my drill press from 1600 to 2400 RPM as the drill bit tore the back of the Paxolin last time. The higher speed helped the drill bit to exit cleanly. The circuit board was already the right width, so all I had to do was trim the end off and sand it. I then peeled off the adhesive paper which came off very easily. I used a 3M sanding sponge to smooth the edges as they were sharp to the touch. There, now it’s perfect!

Perf Board Redesign

Already? I just started! I decided that I want to go with a perf board that uses 7 hole spacing between the component leads instead of 8. I measured this out and it looked too skinny. So instead of subtracting another hole I added another one and I will use these holes for cable strain relief. Of course I will have to re-drill a new circuit board, but I was not entirely happy with the one I just did. Here is the new design. jcm800_017

Perf Board Cutting and Drilling

I used a Bosch jigsaw to trim the circuit board to the right size. I then sanded the edges to make them smooth using my bench top as a sanding block to keep the edge square. I taped the template onto the circuit board with clear packing tape. Using my ultra cheap drill press I purchased from Harbor Freight and a #33 drill bit, I drilled the 256 holes. Some of the Paxolin material got caught under the holes of the paper template causing some of the holes to be slightly off. I recall that the last board I drilled I used adhesive paper for the template but this board is too wide (12″) for a printout on a single sheet of letter paper. I will probably give this another shot even though it takes forever, my O.C.D. will not allow for holes that are slightly off.

Perf Board Design

I like the look of a fully perforated circuit board. I know you Marshall purists out there are cursing my name as true JCM800s used a printed circuit board. But, as I stated on my information page this is not a replica amp.

I created a drilling template using a CAD program called Concept Draw and decided on a matrix of 8 holes by 32 holes for the 50 watt 2204 circuit I am using. I printed out the template, cut it out, and placed it on a sheet of Paxolin. I scored around the template to create cut lines to trim the circuit board down to the proper size.

Paxolin Circuit Board Material

I am going to get pretty crazy about parts for this amp. I have O.C.D. and when I make things they must be exactly what I want. I have been on the Metroamp forum since I started building amps and there always seems to be somebody selling cool parts. I was shopping around for circuit board material and found a forum member that had a stack of brown Paxolin circuit board material that he was selling for cheap. He ordered a bunch from England and didn’t need the rest. I ended up getting 8 sheets for $30 shipped which is an amazing deal. My clone starts it’s life with the perfect circuit board material from the motherland of Marshall Amplification.